A calculation of neutron cross-sections for the elements hydrogen, carbon, nitrogen and oxygen in the energy range 20 - 50 MeV

The advent of high energy neutron radiotherapy has created a need for an evaluated set of neutron cross-section data extending up to 50 MeV for the main constituents of tissue, ie, hydrogen, carbon, nitrogen and oxygen. The experimental data between 20 and 50 MeV are so few that nuclear model calculations have to be employed to produce the required cross-sections (except for hydrogen, where a semi-empirical form of the effective range theory can be used to represent the data). The two nuclear models which have been employed are the optical model and the statistical model of level densities. The strategy has been to use the optical model to fit the available experimental total cross-section data and in so doing provide values for the nonelastic cross-sections which can then be used to normalise the individual reaction cross-sections calculated by the statistical model. Total and elastic cross-sections calculated by the optical model and reaction cross-sections produced by the statistical model for carbon, nitrogen and oxygen are presented and where possible compared with experimental data. (author)